Ultrasound Basics

The goal of POCUS is to augment your physical exam by peering inside the body and looking directly at the organs themselves, instead of only making assumptions based on sounds and labs. Let's start with a very brief primer on ultrasound basics so you understand how to successfully acquire an image.

Choice of probe

Phased Array

1-5 MHz, up to 35cm depth

The workhorse probe that can do the majority of imaging. Use this to look at the heart, lungs, abdomen, and pretty much anything else!

Linear

5-10 MHz, up to 9cm depth

Excellent for shallow structures like veins/arteries, subcutaneous tissue. Also very useful for looking closer at the pleura, eyes.

Curvilinear

2-5 MHz, up to 30cm depth

Broad & trapezoidal ultrasound beam. Long wavelengths that penetrate to deep tissues. Great for abdominal organs.

Orientation & Probe Marker

Each probe has a marker on one side (blue dot) that will correspond to the marker on the screen. For most imaging, the probe marker has been arbitrarily designated to always be on the left. The exception to this rule is cardiology convention, when the probe marker is on the right of the screen when ultrasounding the heart.

In this lung imaging example below, the probe marker is facing the head (black arrow) which corresponds to the blue dot on the top left of the ultrasound image. Therefore the left of the ultrasound image is superior (towards the head), while the right side of the image is inferior (towards the feet).

P-DAG the image

For every image interpretation, the first step is always P-DAG.

Probe Marker:
It is very important to first confirm that the probe marker on the screen is correctly oriented with the probe marker on your probe. For all organ systems (apart from cardiac), the probe marker should be on the left side of the image (except for emergency medicine where the probe marker is always on the left - including cardiac imaging).

Depth:

The object you are targeting should be in the center of the screen. It is important to always start with a deeper view to look behind the organ of interest, and then reduce the depth until your target is centered.

Axis:

The axis of the image is the plane in which you are imaging. This becomes especially important in cardiac imaging, as being 'off axis' can give you cuts of the heart that can be very misleading. For example, in the first view of the heart,  Parasternal Long Axis, the left ventricle should be football shaped. The apex will usually not be seen and the mitral valve should be central.

Gain:

Gain, or brightness of the image will vary with each patient and each structure.

Too Deep

In this example, the depth is set to 21 cm. If you are evaluating the heart, it is clearly not in the center of the image, and the top portion of the right ventricle cannot be seen. You would need to reduce the depth to about 15cm to center the image. However as discussed above, it is important to start very deep otherwise you may miss something - in this case, a left pleural effusion seen deep to the heart.

Off Axis

In this parasternal long view, the image is off axis. You can tell because the left ventricle is not the typical football shape, and the walls are collapsing with each beat. This image could cause you to overestimate the ejection fraction.

Overgained

The gain, or brightness, of this image is set too high. The pericardium is lighting up and overpowering the image. Also the area that should be hypoechoic inside the chambers that are filled with blood have a brighter appearance because of the excess gain. 

POCUS Med Ed

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